The main vibrations of an internal combustion engine are not caused by the combustion of the mixture in the cylinders (even if the crankcase is particularly rigid) and are not even caused by the rotation of the crankshaft (which is balanced by means of specific counterweights or special shapes), but are determined by the reciprocating motion of the pistons which accelerate under the bias of the gases, slow down suddenly and pick up speed again reversing their direction of motion, i.e. are determined by the so-called reciprocating inertial forces. The piston causes the most violent shuddering (i.e. the maximum intensity of the reciprocating inertial forces) when the direction of motion is reversed (i.e. when its acceleration/deceleration is maximum), but reciprocating inertial forces displaying variable intensity over time with a sinusoidal law are present in all instants of motion.
Reciprocating inertial forces generate vibrations which are detrimental for the engine (because they can cause failure by fatigue over time) and are also annoying for vehicle occupants. In order to reduce such vibrations, the internal combustion engine must be balanced, i.e. by using balancing countershafts (also known as balancing shafts), which allow to reduce (compensate) the reciprocating inertial forces. However, the use of balancing countershafts gives rise to various drawbacks due to the weight and dimensions of the balancing countershafts, the weight and dimensions of the drive needed to turn the balancing countershafts and the greater weight of the engine crankcase which must also support the stress induced by the balancing countershafts (the balancing countershafts generate further reciprocating inertial forces which are transmitted to the engine crankcase and cancel out the reciprocating inertial forces of the pistons in the engine crankcase itself).
U.S. Pat. No. 4,517,933A1 describes an internal combustion engine with a “V” configuration of the cylinders, wherein the crankshaft is provided with balancing masses 31a, 31b and 31c to reduce the vibrations generated by the reciprocating motion of the pistons.